With reference to two monomers (i.e., of a set of at least two monomers that are polymerized to produce a copolymer), the term “comonomer” is herein used to denote the monomer that decreases the copolymer's density if its relative amount in the copolymer increases. Conversely, with reference to the same two monomers, the term “monomer” (rather than “comonomer”) is used to denote the one of these monomers that increases the copolymer's density if its relative amount in the copolymer increases. For example, when hexene and ethylene are polymerized (typically in the presence of a catalyst system) to produce polyethylene, the ethylene is herein referred to as the “monomer” and the hexene is herein referred to as the “comonomer” (since increasing the relative amount hexene in the polyethylene decreases the density of the polyethylene).
Throughout this disclosure, the expression polyethylene denotes a polymer of ethylene and optionally one or more C3-C10 alpha olefins (said alpha olefins being comonomers) and the expression polyolefin denotes a polymer of one or more C2-C10 alpha olefins, preferably alpha olefins.
Throughout this disclosure, the phrase “off-grade product” (e.g., “off-grade” polymer resin) assumes that the product is produced in a reactor with the intention that it meet a specification set (a set of one or more specifications for one or more properties of the product) and denotes that the product has at least one property that does not meet at least one specification in the specification set. For example, if the specification set requires that the product have a resin flow property (e.g., melt index) within a specified first range and a density within a specified density range, the product is an off-grade product if its resin flow property is outside the first range and/or its density is outside the density range.
A product (e.g., an off-grade product) having “excessively low” density herein denotes a reaction product having density below the low end of a density range specified by a specification set. With reference to an initial polymerization reaction designed to produce a product with properties meeting an initial specification set (including an initial density range for the product) and a target polymerization reaction designed to produce a product with properties meeting a target specification set (including a target density range for the product), the expression that a product has “excessively low density” herein denotes that the product has density below both the initial density range and the target density range.
With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. The most recently produced quantity typically undergoes mixing with previously produced quantities of the product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
Throughout this disclosure, the abbreviation “MI” denotes melt index and the abbreviation “FI” denotes flow index.
One commonly used method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor, during operation to produce polyolefins by polymerization, contains a fluidized dense-phase bed including, for example, a mixture of reaction gas, polymer (resin) particles, catalyst, and co-catalyst. Typically, any of several process control variables can be controlled to cause the reaction product to have desired characteristics.
Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of a catalyst. This gaseous stream is withdrawn from the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer and/or comonomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is removed in another part of the cycle by a cooling system external to the reactor.
The amount of polymer produced in a fluidized bed polymerization process is directly related to the amount of heat that can be withdrawn from the fluidized bed reaction zone since the exothermic heat generated by the reaction is directly proportional to the rate of polymer production. In steady state operation of the reaction process, the rate of heat removal from the fluidized bed must equal the rate of heat generation, such that the bed temperature remains substantially constant. Conventionally, heat has been removed from the fluidized bed by cooling the gas recycle stream in a heat exchanger external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the resin and gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Heat removal is also important to prevent excessive stickiness of polymer particles that if left unchecked, can result in agglomeration of the sticky particles which can in turn lead to formation of chunks or sheets of polymer that cannot be removed as product (e.g., sheets that cause dome or wall sheeting) and can cause loss of fluidization. Such resin chunks or sheets may fall onto the reactor's distributor plate causing impaired fluidization, and in many cases forcing a reactor shutdown (a “discontinuity” event). Prevention of resin stickiness has been accomplished by controlling the temperature of the fluid bed to a temperature below the fusion or sintering temperature of the polymer particles. Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to resin agglomeration or stickiness, which in turn can, if left unchecked, lead to formation of resin chunks or sheets and impaired fluidization.
It is known that production of polymer resin having excessively low density during resin-producing polymerization reactions in fluidized-bed, gas phase reactors is typically undesirable because such resin can be or become sticky at normal reaction temperatures, and that such resin stickiness during the reaction can cause a reactor discontinuity event (e.g., due to sheeting or chunking as mentioned above).
A change from production of one grade of polymer to another typically requires a transition period for a polymerization reactor to switch over to a new resin specification set and corresponding process conditions such as reaction temperature, reactants and reactant concentration ratios. During a transition from an initial polymerization reaction intended to produce an initial resin product meeting a first specification set to a target polymerization reaction intended to produce a target resin product meeting a second specification set, off-grade polymer may be produced whose density (or other property) does not meet either the first or the second specification set. For example, during such a transition off-grade polymer having excessively low density (as herein defined) may be produced. Unless the transition is implemented appropriately, such off-grade product may become sticky under the conditions (including temperature) present during the transition, and agglomeration or sheeting (on the reactor wall or dome) as well as product discharge problems can result. Of course, both the initial and target reactions are typically intended to produce resin that will not become sticky under steady state reaction conditions.
In the typical case that a polymerization reaction transition involves changing at least one of: reactor temperature, monomer concentration (e.g., ethylene partial pressure, in the case of polyethylene polymerization) in the reactor, concentration of isopentane or another induced condensing agent in the reactor, concentration of a continuity agent in the reactor, and concentration of hydrogen in the reactor, or any combination of such reaction parameters, it may not be possible to predict a priori the ratio of comonomer gas concentration to monomer gas concentration in the reactor (e.g., the C6 to C2 gas partial pressure ratio when the reaction is a polyethylene polymerization reaction) or the ratio of feed rates of comonomer and monomer into the reactor that would be required during (or at the end of) the transition to prevent and/or reduce production of product having excessively low density, unless a database regarding each product and process conditions for producing each product is available.
Reactor temperature or monomer concentration changes during such transitions typically cause the solubility (in the polymer) of the reactants and inert materials or induced condensing agents (e.g., isopentane) in the reactor and the reactivities of the reactants to vary. Due to such changes in thermodynamics and kinetics, it is typically not possible to know the precise value at which the concentration of comonomer in the reactor or the feed rate of comonomer flowing into the reactor should be maintained, either during or at the end of the transition. Thus, conventional control of this type of transition typically includes the steps of choosing either the ratio of comonomer concentration to monomer concentration in the reactor (e.g., C6/C2 concentration ratio) or the comonomer to monomer feed ratio (e.g., C6/C2 feed ratio) and maintaining the chosen ratio at a constant value (i.e., the value at the start of the transition) while implementing the process changes required to bring the produced polymer into compliance with the target (post-transition) specification set. If the correct ratio is selected, the transition can proceed as desired without a discontinuity event. Often however, the correct ratio is not known, and as a result the conventionally controlled transition produces excessively low density polymer which often causes various associated problems and even undesired reactor shut down (e.g., due to sheeting caused by resin stickiness). In contrast with this type of conventional control, there exists a need to reliably avoid or reduce production of excessively low density polymer during transitions and thus avoid associated problems.
Background references include U.S. Pat. No. 6,649,710, U.S. Patent Application Publication Nos. 2005/267267, 2007/073010, WO 00/32651, WO 03/044061, and WO 2005/049663.